What is a floodway Zone

These guidelines aim to ensure that all the design criteria have been considered for designing a floodway specifically from a road geometric design perspective. Guidelines for hydrological and hydraulics analysis and structural and civil engineering design of a floodway can be found in Main Roads Floodway Design Guide.

Cookies are required in order to sign in. Please enable cookies in your browser's settings and disable any adblockers. Then refresh this page.

The Government of Western Australia acknowledges the traditional custodians throughout Western Australia and their continuing connection to the land, waters and community. We pay our respects to all members of the Aboriginal communities and their cultures; and to Elders both past and present.

FEMA floodway Map

The extent of cement stabilised pavement should be extended to 1% AEP water level or vertical crest VPI, whichever is lower.

Floodway is designed such that it can be trafficable for all flows up to the design AEP and for all flows up to 2% AEP can be contained within the floodway approach without spilling out elsewhere along the road.

Floodway vs floodplain

A relief culvert should be placed at the lowest point of the natural surface to drain the perennial or frequent flows. This prevents water from ponding upstream of the floodway, softening the embankment and subgrade which ultimately causes pavement failure.

The extent of embankment scour protection should be extended beyond the floodway design length to the water level extent as shown in Figure 2. Consideration should be given to minimising the closure time by ensuring the road is not damaged after the design rainfall event. Road damage repair work is always difficult in a remote area and the fund for road repair work may not always be available for road damage caused by frequent events. The recommended desirable Annual Exceedance Probability (AEP) water level for the extent of embankment scour protection is 2%.

FEMA floodway

Design guidelines for pavement and embankment scour protection for floodways can be found in Main Roads Floodway Design Guide.

Sag vertical curves should be kept short in order not to encroach on the waterway in the floodway depression and not to increase the raised portion of the ramp more than is strictly necessary. Road embankments on the approaches to floodways are susceptible to scour and their length should be kept as short as possible. The angle of the ramp should therefore be as high as possible and consistent with other geometric requirements. A combination of 40m sag vertical curve with 0.8% longitudinal grade on floodway ramp is typically used, however, it should be noted that this combination will result in a very marginal deficiency in headlight criteria for a design speed of 110km/h with a reaction of 2.5 seconds. In this case, the length of sag vertical curve is expected to be increased slightly in order to meet the standard headlight criteria.

Floodway Map

Floodways are commonly used on rural roads with relatively low traffic volume and where it is impractical or uneconomical to construct a bridge or culvert.The road geometric vertical alignment components of a floodway are outlined in Figure 1. The length of the floodway is generally taken to be the length between the intersection points of sag vertical curves, no account being taken of the extra capacity of floodway approaches or loss of capacity due to the sag vertical curves.

Floodway should be on a straight horizontal alignment. When a floodway is on a horizontal curve and overtopped by water, the risks are listed below. Floodway and floodway approaches should not be located in either horizontal curves or plan/superelevation transitions.

There was an error during authentication. Please refresh to try again. If this issue persists, please sign out and sign back in after reloading.

In general, there are 5 typical floodway types and their guideline drawings are listed in Table 1. Each of the types varies in some ways from the others and their applications are explained in Main Roads Floodway Design Guide.